For examination of objects of interest with electromagnetic radiation, visible or invisible light or X-rays may be used. X-ray differential phase-contrast imaging (DPCI) visualizes the phase information of coherent X-rays passing a scanned object. In addition to classical X-ray transmission imaging, DPCI determines not only the absorption properties of a scanned object along a projection line, but also the phase shift of the transmitted X-rays, and thus provides variable additional information usable for contrast enhancement, material composition or dose reduction.
Recently, a group at Paul-Scherrer Institute, Villingen, Switzerland has introduced a realization of DPCI (see for example EP 1 731 099 A1, EP 1 879 020 A1, Pfeiffer et al., Nature Physics 2, 258 (2006).
While older differential or non-differential PCI methods may suffer from the requirement of highly monochromatic and coherent X-ray sources, the above method may allow the use of standard X-ray sources, i.e. X-ray tubes, with an additional source grating which may assure coherence through small openings. After the object of interest to be imaged, a phase-shifting grating (G1) is placed (working as a “beam splitter”). The resulting interference pattern (see FIG. 2) contains the required information about the beam phase shift in the relative position of its minima and maximal (typically in the order of several micrometers). Since a common X-ray detector (typical resolution in the order of 150 μm) is not able to resolve such fine structures, the interference is sampled with a phase-analyzer grating (also known as “absorber grating G2”) which features a periodic pattern of transmitting and absorbing strips with the periodicity similar to that of the interference pattern.
The similar periodicity produces a Moire pattern behind the grating with a much larger periodicity, which is detectable by a common X-ray detector (see FIG. 3). To obtain the differential phase shift, the analyzer grating (absorber grating) G2 needs to be shifted laterally by fractions of the grating pitch p (typically of the order of 1 μm), referred to as “phase stepping”. The phase shift can be extracted from the particular Moire pattern measured for each position of the analyzer grating. In an extension, computed tomography of phase-shift with hard X-rays may also be performed.
However, in particular in the case of cone-beam geometry, strong phase-contrast distortions may arise in regions outside the centre of the field of view (FOV).